Oligomerization of isocyanates is a long-known, generally accepted method of modifying low molecular weight isocyanates, which are usually difunctional, in order to obtain products with advantageous application properties e.g. in the paint and coating sector; these will be referred to generally as polyisocyanates in this specification (J. Prakt. Chem./Chem. Ztg 1994, 336, 185–200).
Polyisocyanates based on aliphatic diisocyanates are normally used for light-resistant, non-yellowing paints and coatings. The term “aliphatic” refers to the carbon atoms to which the NCO groups of the monomer are bonded, i.e. the compound molecule may perfectly well contain aromatic rings, which do not then of course carry NCO groups.
One can distinguish between different products and processes according to the type of structure mainly formed from the previously free NCO groups in the respective oligomerization reaction.
Particularly important procedures are so-called dimerization to form uretdione structures of formula 1, described for example in DE-A 16 709 720 and so-called trimerization to form isocyanate structures of formula 2, described for example in EP-A 0 010 589. In addition to the last-mentioned trimers isomeric, i.e. also trimeric products with an iminooxadiazindione structure of formula 3 can be obtained as described for example in EP-A 0 798 299 on isocyanurates. If this specification refers to both isomeric trimers, isocyanurates and iminooxadiazindiones, it will generally be speaking of trimers or trimerized compounds, otherwise the exact term will be used. The term “oligomerization” covers all types of modification.
In addition to the products providing the name of the reaction (dimer for dimerization, trimer for trimerization) the other respective reaction products are almost always also produced simultaneously during the dimerization and trimerization of isocyanates: trimers of formulae 2 and 3 during the dimerization and uretdiones of formula 1 during the trimerization, the content of which, however, is low in each case:
X=difunctional substituent
Complete conversion of all monomeric diisocyanate molecules OCN—X—NCO in one reaction step would lead to high molecular weight, extremely high-viscosity or gel-like products which would be useless in the paint and coatings sector, owing to further reaction of the NCO groups in formulae (ideal structures) 1 to 3. In catalyzed preparation of polyisocyanates for paint the industrial procedure is therefore to convert only part of the monomer, to stop any further reaction by adding a catalyst poison (a “stopper”) and then to separate the non-converted monomer. The aim is to have to separate the smallest possible proportion of non-converted monomer at the lowest possible viscosity of the low-monomer polyisocyanate paint resin, i.e. to obtain high conversion in the reaction accompanied by a high resin yield at the following processing stage with high-level properties of the polyisocyanate resins.
Dimers based on aliphatic diisocyanates have a far lower viscosity than trimers. However they have a strictly linear, i.e. NCO-difunctional structure regardless of the degree of conversion or the resin yield. Trimers on the other hand have the higher functionality required for a high crosslink density in the polymer and consequent good stability properties thereof. Their viscosity increases very rapidly though with increasing conversion in the reaction. Compared with isomeric isocyanurates iminooxadiazindiones have far lower viscosity with the same NCO-functionality of the polyisocyanate resin (cf. Proc. of the XXIVth Fatipec Conference, Jun. 8–11, 1998, Interlaken, CH, vol. D, pp. D-136–137), though they do not reach the viscosity level of uretdiones.
State of the art for producing polyisocyanates of the trimeric type is isocyanate oligomerization using a large number of both saline and covalently structured catalysts (J. Prakt. Chem./Chem. Ztg. 1994, 336, 192 to 196 and literature quoted therein). While very small quantities of catalyst are sufficient for isocyanate oligomerization when using compounds with a saline structure, such as carboxylates (for example DE-A 3 100 263), fluorides (for example EP-A 339 396) or hydroxides (for example EP-A 330 966) and the desired rate of conversion is achieved in a very short time, higher catalyst concentrations and/or prolonged reaction times are required when using covalently structured trimerization catalysts. An example of this is the oligomerization of aliphatic diisocyanates with N-silyl compounds, described, for example in EP-A 57 653, EP-A 89 297, EP-A 187 105, EP-A 197 864 and WO 99/07765.
Up until now, just covalently structured catalyst systems have been described for producing polyisocyanates with uretdione structure (J. Prakt. Chem./Chem. Ztg. 1994, 336, 196 to 198 and literature quoted therein). Most widespread are trialkylphosphines (described inter alia in DE-A 1 670 720) and in pyridines amino substituted in the 4-position (described inter alia in DE-A 3 739 549).
The disadvantage of the method of the state of the art is that, on the one hand, highly active, catalysts with a saline structure are virtually exclusively capable of generating trimers but rarely of forming uretdione and the uretdione selective/uretdione-more selective catalysts are all covalently structured, for which reason they have to be used in comparatively high concentrations, based on the mass of catalyst and isocyanate to be oligomerized, and also only lead to relatively slow progress of the reaction. Both of these factors are disadvantageous in terms of cost efficiency (space/time yield during production) and paint technology (disruptive influences of catalyst and/or catalyst secondary product in the polyisocyanate).
It is an object of the present invention to provide a catalyst system for isocyanate oligomerization which is saline in structure and therefore highly reactive but nevertheless leads to the formation of significant uretdione contents in the resulting polyisocyanate.
The above-described object has been achieved by the use of saline derivatives of five-membered N-heterocycles which carry at least one hydrogen atom bound to a ring nitrogen atom in the neutral molecule, as catalyst for isocyanate oligomerization.
The invention is based on the surprising observation that saline derivatives of five-membered N-heterocycles, which carry at least one hydrogen atom bound to a ring nitrogen atom in the neutral molecule, catalyze isocyanate oligomerization and that uretdione structures are also formed to a considerable extent in the method in addition to isocyanate trimers.
Nitrogen heterocycles are already used in polyisocyanate chemistry as neutral, N—H- or N-alkyl group-carrying compounds. However, they are generally used as blocking agents for NCO groups (NH-group-containing derivatives, cf. EP-A 0 741 157) or as stabilizers to prevent UV radiation-induced damage to the paint film produced from the polyisocyanates, for example, substituted benzotriazoles which contain further OH groups in the molecule, cf. for example DE-A 198 28 935, WO 99/67226 and literature quoted therein.
In the aforementioned fields of application, the aim is not oligomerization of the isocyanate groups, rather their thermally reversible deactivation to enable single component processing or stabilization of the polyurethane plastics material or paint. Oligomerization of the isocyanate groups would even be disadvantageous in both cases.
Furthermore, references are occasionally made in the patent literature to the use of N-heterocycles as additive to influence the catalytic activity of certain catalysts, as catalyst itself or else to suppress undesirable effects such as increase in colour index etc. Therefore, WO 99/23128 describes a system, containing inter alia a “trimerization catalyst” and imidazole. However, again only the neutral compound of the nitrogen heterocycle is used and not the anion. From the examples in WO 99/23128 it emerges that imidazole is added to the isocyanate to be oligomerized, before trimerization, for which reason the above-mentioned addition reaction to the NCO groups of the isocyanates to be modified initially takes place and therefore no “in situ” formation of the imidazolate-anion could be initiated even subsequently, after addition of the “trimerization catalyst”.
The dimerization or trimerization of benzylisocyanate under the influence of 1,2-dimethylimidazole is also described in Adv. Ureth. Sci. Techn. 1971, 1, 33 and in Synthesis, 1975, 463. Anions of the heterocycles are not mentioned in the cited documents. There is also no option of an in situ generation of anionic species owing to the absence of an acid H atom in the 1,2-dimethylimidazole. The same applies to the method in EP-A 417 603, EP-A 566 247, EP-A 672 696 and EP-A 982 333, which refer exclusively to heterocycles carrying N-alkyl groups in which generation of anionic species is not possible for the reasons given above.